The factory in which/where his father worked has closed.
The factory which was built in 1978 has closed.
第一个例子,the factory 是先行词,从句中的work为不及物动词,the factory做从句的状语,所以可以用关系副词where 表地点,也可以用in which (因为work in the factory)
第二个例子,the factory 是先行词,从句缺主语,the factory 做从句的主语,用关系代词（which/that)
所以分清先行词在从句中所做的句子成份很重要（做主语、宾语用关系代词；做状语用关系副词）
关系代词：that/which/who/whom/whose
关系副词：when,why,where
介词+which
灵活运用就OK了.